How to append multiple materials from different blend files
In Blender 2.76 you'll be able to set how many directory levels to show simultaneously.

You can download the last build, though it was not as stable as release:
https://builder.blender.org/download/

It works both for system folder and blender file data



Let's say your blend files are all in the same directory.

Choose One Level and filter by material:



You'll find all the materials used in all the blend files.